Wet-strength finishing agents for paper

ABSTRACT

They are mixtures of  
     (a) from 1 to 99.9% by weight of a water-soluble epihalohydrin-crosslinked polyamidoamine and  
     (b) from 0.1 to 20% by weight of at least one other cationic polymer.

DESCRIPTION

[0001] The present invention relates to wet strength enhancers for paperand to a process for producing wet-strengthened enhanced paper. U.S.Pat. No. 2,926,154 discloses water-soluble reaction products of anepihalohydrin and polyamidoamines. The reaction products are used inpapermaking by adding them to the paper stock as wet strength agents.

[0002] WO-A-98/32798 discloses a polymer combination prepared bycrosslinking a polymer mixture of a polyamidoamine and a vinylaminepolymer with an epihalohydrin. These reaction products are added to thepaper stock in a papermaking process to add dry and wet strength to thepaper.

[0003] U.S. Pat. No. 4,880,497 discloses copolymers which containvinylamine units and which are formed by hydrolysis of copolymers ofN-vinylformamide and other ethylenically unsaturated monomers. Thecopolymers containing vinylamine units are added to the paper stock inthe papermaking process to enhance the dry and wet strength of paper.

[0004] The reaction products of the reaction of epihalohydrins withamino-containing compounds have the disadvantage of containing majoramounts of chlorinous by-products.

[0005] It is an object of the present invention to provide improved wetstrength enhancers for paper.

[0006] We have found that this object is achieved according to theinvention by wet strength enhancers for paper, comprising mixtures of

[0007] (a) from 1 to 99.9% by weight of a water-solubleepihalohydrin-crosslinked polyamidoamine and

[0008] (b) from 0.1 to 20% by weight of at least one other cationicpolymer from the group consisting of

[0009] addition polymers containing vinylamine units,

[0010] dicyandiamide-formaldehyde condensates.

[0011] Particular preference is given to wet strength enhancerscomprising

[0012] (a) a water-soluble epichlorohydrin-crosslinked polyamidoamineand

[0013] (b) a 1-100 mol % hydrolyzed polyvinylformamide.

[0014] The invention also provides a process for producing paper bydraining a paper stock in the presence of a wet strength enhancer, whichcomprises using wet strength enhancers comprising mixtures of

[0015] (a) from 1 to 99.9% by weight of a water-solubleepihalohydrin-crosslinked polyamidoamine and

[0016] (b) from 0.1 to 20% by weight of at least one other cationicpolymer from the group consisting of

[0017] addition polymers containing vinylamine units and

[0018] dicyandiamide-formaldehyde condensates.

[0019] In this papermaking process the paper stock is admixed with (a)an epihalohydrin-crosslinked polyamidoamine and (b) at least one othercationic polymer concurrently or in any order, said components (a) and(b) being used in any event in a weight ratio of from 1 to 99.9:0.1 to20.

[0020] The invention further provides for the use of the above-describedwet strength enhancers in the making of paper by addition to the paperstock before sheet formation in amounts of from 0.1 to 4% by weight,based on dry fiber.

[0021] Component (a) of the paper wet strength enhancers according tothe invention may be a water-soluble epihalohydrin-crosslinkedpolyamidoamine. Polyamidoamines may be prepared by condensation ofdicarboxylic acids with polyalkylenepolyamines, cf. U.S. Pat. No.2,926,154 and WO-A-98/32798. For example, from 0.8 to 1.4 mol of apolyalkylenepolyamine is used per mole of dicarboxylic acid.

[0022] Polyamidoamines are preferably prepared using aliphaticdicarboxylic acids having 2 to 10 carbon atoms, for example oxalic acid,malonic acid, succinic acid, maleic acid, glutaric acid, adipic acid,azelaic acid and lauric acid. Preferred dicarboxylic acids are adipicacid and glutaric acid.

[0023] Examples of polyalkylenepolyamines are diethylenetriamine,tripropylenetetramine, tetraethylenepentamine,methyl-bis-(3-aminopropyl)amine, diaminopropylethylenediamine,bisaminopropylethylenediamine and aminopropylethylenediamine.

[0024] The condensation of dicarboxylic acids withpolyalkylenepolyamines is effected at elevated temperature, for exampleat from 110 to 220° C. The water formed in the course of thecondensation is distilled out of the reaction mixture. The condensationmay also be effected in the presence of lactones or lactones ofcarboxylic acids having 4 to 8 carbon atoms. The reaction withepihalohydrins, preferably epichlorohydrin, is effected in aqueoussolution at for example from 20 to 100° C., preferably from 30 to 80° C.The reaction of polyamidoamines with epihalohydrins is carried on onlyto that point at which the resultant reaction products remain dissolvedin water. Once the viscosity of the reaction solution has attained thedesired value, the reaction is terminated by addition of an acid, forexample acetic acid or formic acid. This provides aqueous solutions ofan epichlorohydrin-crosslinked polyamidoamine having a viscosity of forexample from 50 to 2000 mPas, preferably from 60 to 400 mPas (determinedin a Brookfield viscometer at 20° C., spindle 2, 20 revolutions perminute, concentration of the aqueous polymer solution: 12.5% by weight).

[0025] Useful cationic polymers may be derived for example fromsynthetic and natural cationic polymers. Useful natural polymers includefor example cationic polysaccharides, cationic starch, cationic amyloseand derivatives thereof, cationic amylopectin and derivatives thereofand also cationic guar derivatives.

[0026] Synthetic cationic polymers include for examplepolyethyleneimines. They may be prepared by polymerizing ethyleneiminein aqueous solution in the presence of acid-detaching compounds, acidsor Lewis acids. Polyethyleneimines are commercially available and theirmolar masses range for example from 200 to 2 000 000, preferably from200 to 1 000 000. The process of the invention particularly preferablyutilizes polyethylenimines having molar masses from 500 to 800 000.

[0027] A further class of synthetic cationic compounds is that of theaddition polymers containing vinylamine units. They may be prepared fromopen-chain N-vinylcarboxamides of the formula

[0028] where R¹ and R² are identical or different and are each selectedfrom the group consisting of hydrogen and C₁-C₆-alkyl. Useful monomersinclude for example N-vinylformamide (R¹═R²═H in formula I),N-vinyl-N-methylformamide, N-vinylacetamide, N-vinyl-N-methylacetamide,N-vinyl-N-ethylacetamide, N-vinyl-N-methylpropionamide andN-vinylpropionamide. The polymers may be prepared by polymerizing themonomers mentioned alone, mixed with each other or together with othermonoethylenically unsaturated monomers. The addition polymers inquestion are preferably homo- or copolymers of N-vinylformamide.

[0029] Useful monoethylenically unsaturated monomers forcopolymerization with N-vinylcarboxamides include all compoundscopolymerizable therewith. Examples thereof are vinyl esters ofsaturated carboxylic acids of 1 to 6 carbon atoms such as vinyl formate,vinyl acetate, vinyl propionate and vinyl butyrate. Useful comonomersfurther include ethylenically unsaturated C₃-C₆-carboxylic acids, forexample acrylic acid, methacrylic acid, maleic acid, crotonic acid,itaconic acid and vinyl ester acid and also their alkali metal andalkaline earth metal salts, esters, amides and nitriles of thecarboxylic acids mentioned, for example methyl acrylate, methylmethacrylate, ethyl acrylate and ethyl methacrylate. Further usefulcarboxylic esters are derived from glycols or polyalkylene glycols wherein each case only one OH group is esterified, for example hydroxyethylacrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate,hydroxybutyl acrylate, hydroxypropyl methacrylate, hydroxybutylmethacrylate and also monoacrylate esters of polyalkylene glycols havinga molar mass of from 500 to 10 000. Useful comonomers further includeesters of ethylenically unsaturated carboxylic acids with aminoalcohols,for example dimethylaminoethyl acrylate, dimethylaminoethylmethacrylate, diethylaminoethyl acrylate, diethylaminoethylmethacrylate, dimethylaminopropyl acrylate, dimethylaminopropylmethacrylate, diethylaminopropyl acrylate, dimethylaminobutyl acrylateand diethylaminobutyl acrylate. Basic acrylates can be used in the formof the free bases, salts with mineral acids such as hydrochloric acid,sulfuric acid or nitric acid, the salts with organic acids such asformic acid, acetic acid, propionic acid or of sulfonic acids or inquaternized form. Useful quaternizing agents include for exampledimethyl sulfate, diethyl sulfate, methyl chloride, ethyl chloride orbenzyl chloride.

[0030] Useful comonomers for the monomers of the formula I furtherinclude amides of ethylenically unsaturated carboxylic acids such asacrylamide, methacrylamide and also N-alkylmonoamides and diamides ofmonoethylenically unsaturated carboxylic acids with alkyl radicals offrom 1 to 6 carbon atoms, for example N-methylacrylamide,N,N-dimethylacrylamide, N-methylmethacrylamide, N-ethylacrylamide andN-propylacrylamide and tert-butylacrylamide and also basic(meth)acrylamides, for example dimethylaminoethylacrylamide,dimethylaminoethylmethacrylamide, diethylaminoethylacrylamide,diethylaminoethylmethacrylamide, dimethylaminopropylacrylamide,diethylaminopropylacrylamide, dimethylaminopropylmethacrylamide anddiethylaminopropylmethacrylamide.

[0031] Useful comonomers for the monomers of the formula I furtherinclude N-vinylpyrrolidone, N-vinylcaprolactam, acrylonitrile,methacrylonitrile, N-vinylimidazole and also substitutedN-vinylimidazoles, for example N-vinyl-2-methylimidazole,N-vinyl-4-methylimidazole, N-vinyl-5-methylimidazole,N-vinyl-2-ethylimidazole and N-vinylimidazolines such asN-vinylimidazoline, N-vinyl-2-methylimidazoline andN-vinyl-2-ethylimidazoline. N-Vinylimidazoles and N-vinylimidazolinesare used not only in the form of the free bases but also afterneutralization with mineral acids or organic acids or afterquaternization, a quaternization being preferably effected with dimethylsulfate, diethyl sulfate, methyl chloride or benzyl chloride. Alsouseful are diallyldialkylammonium halides, for examplediallyldimethylammonium chlorides.

[0032] Useful comonomers for N-vinylcarboxamides further includesulfo-containing monomers, for example vinylsulfonic acid, allylsulfonicacid, methallylsulfonic acid, styrenesulfonic acid, the alkali metal orammonium salts of these acids or 3-sulfopropyl acrylate.

[0033] The copolymers contain for example

[0034] from 99.99 to 1 mol % of N-vinylcarboxamides of the formula I and

[0035] from 1 to 99 mol % of other monoethylenically unsaturatedmonomers copolymerizable therewith

[0036] in copolymerized form.

[0037] To prepare vinylamine polymers it is preferable to start fromhomopolymers of N-vinylformamide or from copolymers obtainable bycopolymerization of

[0038] N-vinylformamide with

[0039] vinyl formate, vinyl acetate, vinyl propionate, acrylonitrile orN-vinylpyrrolidone

[0040] and subsequent hydrolysis of the homo- or copolymers to formvinylamine units from the copolymerized N-vinylformamide units, thedegree of hydrolysis being for example in the range from 1 to 100 mol %.For instance, polyvinylamine is obtained by complete hydrolysis (degreeof hydrolysis 100 mol %) of homopolymers of N-vinylformamide.

[0041] The hydrolysis of the above-described polymers is effectedaccording to known processes by the action of acids, bases or enzymes.This converts the copolymerized monomers of the above-indicated formulaI through detachment of the group

[0042] where R² is as defined for formula I, into polymers which containvinylamine units of the formula

[0043] where R¹ is as defined for formula I.

[0044] The homopolymers of the N-vinylcarboxamides of the formula I andtheir copolymers may be hydrolyzed to an extent in the range from 1 to100 mol %, advantageously to an extent in the range from 5 to 100 mol %,preferably to an extent in the range from 10 to 100 mol %. In mostcases, the degree of hydrolysis of the homo- and copolymers is in therange from 20 to 95 mol %. The degree of hydrolysis of the homopolymersis synonymous with the vinylamine units content of the polymers. In thecase of copolymers containing units derived from vinyl esters, thehydrolysis of the N-vinylformamide units can be accompanied by ahydrolysis of the ester groups with the formation of vinyl alcoholunits. This is the case especially when the hydrolysis of the copolymersis carried out in the presence of aqueous sodium hydroxide solution.Copolymerized acrylonitrile is likewise chemically modified in thehydrolysis, for example converted into amide groups or carboxyl groups.The homo- and copolymers containing vinylamine units may optionallycontain up to 20 mol % of amidine units, formed for example byintramolecular reaction of an amino group with an adjacent amide group,for example of copolymerized N-vinylformamide.

[0045] Polymers containing vinylamine units also include hydrolyzedgraft polymers of N-vinylformamide on polysaccharides, polyalkyleneglycols and polyvinyl acetate. The N-vinylformamide units grafted ontothe polymers are converted into the corresponding addition polymerscontaining vinylamine units by hydrolysis to detach formyl groups. Graftpolymers containing vinylamine units are described for example in U.S.Pat. No. 5,334,287, U.S. Pat. No. 6,048,945 and U.S. Pat. No. 6,060,566.

[0046] In an embodiment of the present invention, the cationic polymersare used in the form of salt-free aqueous solutions or low-salt aqueoussolutions containing not more than 5% by weight and preferably not morethan 2% by weight of an inorganic salt. Such salt-free or low-saltsolutions may be prepared by ultrafiltration or by precipitation of theneutral salts with organic solvents such as acetone, methyl ethyl ketoneor alcohols.

[0047] Preferred cationic polymers are

[0048] addition polymers containing vinylamine units.

[0049] The molar mass M_(w) of the cationic polymers is not less than 15000 and is preferably in the range from 50 000 to 10 million. The molarmass M_(w) of the cationic polymers is determined by light scattering.The cationic polymers may have a charge density of at least 1.5 andpreferably from 4 to 15 meq/g (measured at pH 7).

[0050] Useful fibers for producing the pulps include all types customaryfor this purpose, for example mechanical pulp, bleached and unbleachedchemical pulp and paper stocks from all annual plants. Mechanical pulpincludes for example groundwood pulp, thermomechanical pulp (TMP),chemothermomechanical pulp (CTMP), pressure groundwood, semichemicalpulp, high yield chemical pulp and refiner mechanical pulp (RMP). Usefulchemical pulps include for example sulfate, sulfite and soda pulps.Preference is given to using unbleached chemical pulps, which is alsoknown as unbleached kraft pulp. Useful annual plants for producing paperstocks include for example rice, wheat, sugarcane and kenaf. Pulps arealso produced using waste paper alone or mixed with other fibers. Wastepaper also includes coated waste, which, owing to its binder content forcoating and printing ink compositions, gives rise to white pitch.Stickies are due to the adhesives from sticky labels and letterenvelopes, due to adhesive materials from spine gluing and due tohotmelts. The fibers mentioned may be used alone or mixed with eachother.

[0051] The inventive wet strength enhancers comprised of components (a)and (b) contain for example from 0.1 to 20% and preferably from 0.2 to5% by weight of a cationic natural and/or synthetic polymer. The wetstrength enhancers are added to the paper stock in the papermakingprocess in amounts from 0.1 to 5% by weight, preferably from 0.5 to 4%by weight, each percentage being based on dry fiber. However, components(a) and (b) can also be added separately to the paper stock in thepapermaking process, in the above-described ratio. It is possible, forinstance, first to add component (a) to the paper stock and then to addcomponent (b) just ahead of the headbox for example. However, the orderof the components can also be reversed and similarly the two componentscan also be added concurrently through a two-material nozzle or throughtwo separately disposed metering positions, to the paper stock.

[0052] Whereas the wet strength of paper cannot be increased to beyond acertain value by raising the amount of a customary wet strength agent,for example an epichlorohydrin-crosslinked polyamidoamine as percomponent (a), which is added to the paper stock, the wet strengthenhancer of the invention provides a further increase in the wetstrength of the paper.

[0053] The percentages in the examples are by weight. The wet breakinglength was determined according to DIN ISO 3781 following a 15 minuteimmersion in water.

EXAMPLES

[0054] The stock model used was a 3.3 g/l consistency pulp of 100%bleached pine sulfate beaten to 32° SR and having a pH of 7.1. Samplesof this pulp were each admixed with the wet strength agents reported inTable 1 and the mixture obtained in each case was drained on aRapid-Köthen sheet former. The basis weight of the sheets of paper was55 g/m² in each case. The sheets were stored at 110° C. for 5 minutes.The wet tensile strength of the sheets was then determined by the methodindicated above. The materials used and the results obtained therewithare reported in the table.

[0055] Wet strength agent 1: commercially available water-solubleepichlorohydrin-crosslinked polyamidoamine (Luresin® KNU), polymerconcentration 13.5% by weight

[0056] Wet strength agent 2: commercially available water-solubleepichlorohydrin-crosslinked polyamidoamine (Kymene® G 3), polymerconcentration 16% by weight

[0057] PVAm: aqueous solution of polyvinylamine having a molar mass Mwof 400 000 g/mol, polymer concentration 11.8% by weight

[0058] Wet breaking length [m] on use of wet strength agent 1, 2 or PVAmWet Wet Addition to strength strength paper stock agent 1 agent 2 PVAmComparison 1  2% of CP¹⁾  912 m  872 m  480 m Comparison 2  5% of CP1550 m 1492 m 1328 m Comparison 3 10% of CP 2165 m 1935 m 1459 m Example1  5% of CP + 2% 2161 m 2094 m — of PVAm Example 2  5% of CP + 2% 2030 m1972 m — of PVAm

We claim:
 1. Wet strength enhancers for paper, comprising mixtures of(a) from 1 to 99.9% by weight of an epihalohydrin-crosslinkedpolyamidoamine and (b) from 0.1 to 20% by weight of at least one othercationic polymer from the group consisting of addition polymerscontaining vinylamine units, dicyandiamide-formaldehyde condensates. 2.Wet strength enhancers as claimed in claim 1, comprising (a) anepihalohydrin-crosslinked polyamidoamine and (b) a 1-100 mol %hydrolyzed polyvinylformamide.
 3. A process for producing paper bydraining a paper stock in the presence of a wet strength enhancer, whichcomprises using wet strength enhancers comprising mixtures of (a) from 1to 99.9% by weight of an epihalohydrin-crosslinked polyamidoamine and(b) from 0.1 to 20% by weight of at least one other cationic polymerfrom the group consisting of addition polymers containing vinylamineunits and dicyandiamide-formaldehyde condensates.
 4. A process asclaimed in claim 3, wherein the paper stock is admitted with (a) anepihalohydrin-crosslinked polyamidoamine and (b) at least one othercationic polymer concurrently or in any order, said components (a) and(b) being used in any event in a weight ratio of from 1 to 99.9:0.1 to20.
 5. The use of the wet strength enhancers of either of claims 1 and 2in the production of paper by addition to the paper stock before sheetformation in amounts from 0.1 to 4% by weight, based on dry fiber.